If you read part 5 on the Fairbank module you’ll notice that I ended up needing a very long module if I wanted to accurately model a crossing of the San Pedro at Fairbank.
To just model the crossing without any land other than abutment areas would require a module over 28 feet long. That is a little long to fit in that 46″ box and could prove difficult to get through doorways or in the back seat of a compact car.
Now first, we rarely accurately model any structure or terrain because real world distances just overwhelm a model quickly. If I use selective compression the module will be more manageable but first I need to make some corrections.
According to the Digital Elevation Model for Fairbank, the area I want to use for the river crossing is approximately 1350 feet wide with a span of 270 feet. These are rough numbers based on doing distance measurements on the DEM itself. They could easily be plus or minus twenty percent. I used the start of the grading as the outside edge for what would be the trestle. This is probably wider than a bridge built in the 1800s.
Back in the 1800s bridge washouts were common and almost expected. For most people horses and wagons were still the primary mode of transportation. Railroads were nice to have, brought progress, made moving supplies and equipment easier, and made traveling long distances more convenient but people still knew how to move a locomotive or a Cornish pump over a mountain using mules.
It was not until railroads were used for regular commuter service and relied on to deliver supplies that things like washouts were no longer tolerated.
I know that many, if not all, of the bridges on the San Pedro washed out several times during the 1800s and early part of the 1900s. That would tend to say that the bridges were built lower and therefore narrower due to the slope of the land.
If you look at the DEM for the Fairbank area again, I have made four ellipses on the map. The red ellipse is the area I want to model. That is the (Southern Pacific) bridge from Fairbank across the San Pedro toward Nogales and eventually Guaymas. This will be the T&GRR bridge toward Guaymas, now narrow gauge.
The blue ellipse is the area where the Babocomari River joins the San Pedro from the West and Walnut Gulch joins the San Pedro from the East. The green ellipse is the bridge for State Route 82. The Yellow ellipse is another Southern Pacific bridge across the San Pedro. In my scenario the SP, to get to Fairbank, would have built this last bridge. It would therefore be standard gauge.
If you look again at the red, green and yellow ellipses, these are all modified landforms. You will clearly see a channel through the areas. If you look at the rest of the map you’ll notice that the channel is very difficult to pick out. It is more of a flood plain. Now look at the up-river (Southern) side of the bridge toward Nogales. The bridge grading has created a deep channel and back cut the land considerably causing an artificial widening of the riverbed.
For my model the Yellow ellipse and the Green ellipse won’t be in my module anyway. The grading in the red ellipse will be removed and the channel raised to more closely match the surrounding areas of the river.
So how wide is my bridge? It is hard to say. I am going to have to do more research. I know that the bridge over the Colorado was 605 feet and it is a much larger river (but also deeper and more clearly defined). At this point I’m guessing that the channel is probably one to two hundred feet wide and the nominal flood plain around four hundred feet wide. I have copies of David Myrick’s Railroads of Arizona volumes 1-3 on the way but preliminary information says that there is very little about the set of bridges over the San Pedro above Benson.
Till next time.